Spinner spreaders for particulate materials are well known for agricultural and road maintenance applications. Many of the materials to be spread are not always free flowing, such as quarried agricultural lime or sand and salt for winter road maintenance. These materials consist of varying particle sizes and shapes that cause the material particles to interlock. The flow characteristics of these materials are also affected by the handling and storing of the material to be spread. As an example, agricultural lime and sand mixtures for road maintenance applications are stored outdoors and subject to moisture. The moisture adds to material cohesion, changes the natural angle of repose and the spatula angle, and further degrades the flow characteristics.
Materials to be spread are typically deposited on the spinner from a material storage bin via a belt conveyor or drag conveyor. As the material falls from the conveyor to the spinner, its flow is discontinuous because the particles adhere together and fall in slabs or blocks. The phenomena are much like those noted in avalanche studies and landslides where material adheres together and falls suddenly in slumps, blocks, and slabs. The result is inconsistent flow to the spinner. With certain materials and under certain conditions of the material and the application such as slow conveying speeds, material flow to the spinner may at times cease and, when weight of the material overcomes the adhesion or shear strength, the material will fall off the conveyor in large slabs. This changing flow does not permit accurate, consistent and even spreading and also may cause stress on the spreader's hydraulics and/or mechanical systems.
Modern variable rate technology for agricultural spreaders requires that the conveyor adjust its speed to match the needs of predetermined nutrient prescriptions. The requirements for material can vary greatly between prescription grids, and conveyor speeds must adjust accordingly to meter the material. Slow conveyor speeds make the slabbing of material more prevalent and result in loss of precision application.
Prior art in spreaders include hanging chains in the material path at the end of the discharge conveyor and attempt to use the motion of the vehicle to keep material from falling in slabs. This has proven to be inconsistent and not useful for the range of materials to be spread.
Other prior art exists where a reel or flail spins and keeps the material flowing. This art may produce a consistent flow, but the speeds of these devices can cause material to be thrown about and not placed accurately on the spinner. As noted in Rissi U.S. Pat. No. 6,517,281, the accurate placement of material on the spinner of precision spreaders is crucial to the spread pattern. This type of device can also be damaged if foreign debris is mixed in with the material to be spread.
European Patent Application EP1285998 A1 teaches a linear reciprocating device with a frequency of 30 to 130 strokes per minute, a stroke that exceeds the pitch of loosening protrusions, and loosening protrusions that are fixed across the material flow. This device can snag loose debris or block an object that is too large to pass the loosening protrusions which in turn could prevent consistent flow of material. Large objects such as rocks or frozen chunks of material may also damage the device itself. Because the protrusions are frequently in the material path (especially at high conveyor speeds), they can also impede the flow of large quantities of conveyed materials.
Prior art in industrial conveying applications may vibrate the conveyor itself to get material to flow smoothly off the conveyor end. Any vibrators attached to the discharge end of the conveyor could also tend to compact the material in the hopper. Vibrators attached directly to conveyors or bin walls also may tend to fatigue the structure they are attached to and cause equipment failures.
Therefore, limitations of prior art often do not achieve the results needed for today's precision spreading.